<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .header{
            width: 800px;
            height: 200px;
            background-color: blue;
        }
        header{
            width: 800px;
            height: 200px;
            background-color: red;
        }
    </style>
</head>
<body>
    <div class="header"></div>asdasd
    <header>    

    </header>asdasd
</body>
</html>